I don't profess to know the "rules" for tanto randori as it is practised at present, but I remember it as we respected the tanto as a real knife, which should be avoided at all costs!!.....
If the opponent scores a tsuki ari, you were in fact "stabbed" and your opponent has scored a tanto tsuki ari ippon, a "kill" if you like.....
The way we practised was to make correct sugiashi and attempt to catch the tanto hand, which is very difficult, but can be done with lots of practice. Or attempt a shomen ate, gyaku, gedan , ushiro ate attack, much the same way as seen now....
My favourite wrist waza were tenkai kote hineri, tenkai kote gaeshi (shihonage), udegarame, wakigatamae.....
We were made to break from the wrestling which seems to be more common today. That was practised more as randori kyogi and shiai, now termed toshu randori....
Does that help?